Inductor quality (Q) is very often reduced by substrate eddy current, i.e. image current. Similarly, analog devices are very sensitive to noise generated by the underlying silicon substrate, i.e. substrate effects.
U.S. Pat. No. 6,180,995 B1 to Hebert describes a process to etch a trench in a substrate under an inductor.
U.S. Pat. No. 6,313,008 B1 to Leung et al. describes a trench formed by an implant and isotropic etch process.
U.S. Pat. No. 6,326,314 B1 to Merrill et al. describes an inductor process with a trench in a substrate.
U.S. Pat. No. 6,287,931 B1 to Chen describes an inductor process with a trench in an insulating layer.
U.S. Pat. No. 6,057,202 to Chen et al. describes a trench under inductors.
U.S. Pat. No. 5,539,241 to Abidi et al. and U.S. Pat. No. 5,600,174 to Reay et al. each describe inductor processes with trenches underlying the structure.